A closed cell rigid polyurethane foam is a good heat insulating material having excellent moldability and processability, and is in wide use as a heat insulating material in refrigerators, buildings, low temperature warehouses, storage tanks, refrigerator ships, or pipings. The rigid foam has been improved in thermal conductivity year by year, and has at present a value of 0.015 W/mK ( on a commercial basis. Thus, it is said that the rigid polyurethane foam has the smallest thermal conductivity of the heat insulating materials presently used around normal temperatures. Nevertheless, further reduction of thermal conductivity is increasingly demanded.
For the production of a closed cell rigid polyurethane foam, a one shot method is usually employed wherein an A component mainly composed of a polyol, a catalyst, a foam stabilizer and a blowing agent and a B component mainly composed of an organic polyisocyanate are mixed together so that the components react to carry out a foaming process and a curing process in parallel, thereby to form a foam.
Among the blowing agents used in the production of such a closed cell rigid polyurethane foam, trichlorofluoromethane or R-11 is well known. However, the conventional chlorofluorocarbons (CFC's) exemplified by R-11 are chemically stable so that they diffuse into the stratosphere to destroy the ozone layer. As results, the solar ultraviolet radiation is not absorbed by the ozone layer, but it reaches the surface of the earth, and is causing a global environmental problem. For this reason, the use of CFC's has been limited since 1989, and the use of R-11 for the production of polyurethane foam as well.
Under those circumstances, the investigation of a substitute as a new blowing agent for the conventional chlorofluorocarbons has been made, and for example, 1,1-dichloro1-fluoroethane (referred to as HCFC-141b hereinafter) or methylene chloride are put up as a candidate of a substitute for R-11.
However, in general, the heat conductivity of a heat insulating material composed of a closed cell rigid polyurethane foam can not be made smaller than that of a blowing agent used in the production of the foam, so that it is almost impossible to produce a closed cell rigid polyurethane foam having such a small heat conductivity as has been hitherto achieved if a substitute for R-11 must be used as a blowing agent.
Thus, a vacuum heat insulating material has been recently given attention again which is composed of a core material enclosed in a vacuum container of metal-plastic laminate film, as disclosed in Japanese Patent Application Laid-open No. 64-4112.
There are already known two kinds of core materials for use in vacuum heat insulating materials. One is an inorganic material such as perlite, and the other is an organic material such as an open cell polyurethane foam. The inorganic material is inferior in workability to the organic material, and in addition, it is of a high density and cost. On the other hand, an open cell rigid polyurethane foam is free from such problems as above involved in the use of inorganic core material. However, it is required that the open cell rigid polyurethane foam has ceils of very small size in order to retain such a low thermal conductivity as required over a long period of time, since when an open cell rigid polyurethane foam is used as a core material, the thermal conductivity of the resultant vacuum heat insulating material is largely dependent upon the cell size.
As the average cell size is taken as an average of major axis and minor axis of cells of foam, the conventional open cell rigid polyurethane foam has an average cell size of 300-1000 microns. Accordingly, it is necessary to make the inside of the foam reach to such a high level of vacuum of about 0.001 mmHg in order to obtain a high performance vaccum heat insulating material. However, it needs infeasibly much time to make such an open cell rigid polyurethane foam having an average cell size of about 300-1000 microns vacuous to such a high level of vacuum as above stated. Thus, it is necessary that an open cell rigid polyurethane foam has an average cell size of not more than 250 microns to reduce the influence of thermal conduction by gas sufficiently by making the inside of the foam vacuous at a readily attainable level of vacuum of 0.1-0.01 mmHg.
It is also an important requisite that the open cell rigid polyurethane foam has no closed cells. When the foam has the slightest amount of closed cells, the vacuum heat insulating material containing such a foam as a core material is greatly reduced in heat insulating performance if the material has an excellent heat insulation at the initial stage, since the blowing gas enclosed in the closed cells escapes from the cells gradually into the vacuum heat insulating material as time passes so that the pressure inside the vacuum heat insulating material rises accordingly.
For instance, presume that an open cell rigid polyurethane foam has a volume of 1800 cm.sup.3 and closed cells in an amount of 2%, and that a vacuum heat insulating material is manufactured which has a thermal conductivity of about 5 mW/mK by making the inside of the foam reach to a level of vacuum of 0.001 mmHg. The resultant vacuum heat insulating material contains about 36 cm.sup.3 of gas in the closed cells, and the gas gradually diffuses into the vacuous open cells against the diffusion resistance of cell walls of the closed cells, and accordingly the pressure inside the vacuum heat insulating material rises to a level of 15 mmHg, and the thermal conductivity is reduced to about 23 mW/mK or more.
If any, such closed cells are contained concentratedly in the skin layer of open cell rigid polyurethane foam. Thus, from the standpoint of raising the yield rate of products, it is necessary to produce a large block of foam, and make the rate of skin layer to the volume of the block as small as possible as the skin layer is removed from the block when it is put to use. However, the production of a large block of open cell rigid polyurethane foam is attended by a difficulty. Namely, while an open cell rigid polyurethane foam is formed, the resulting foam has a temperature as high as about 200.degree. C. or more inside the foam, and the air readily penetrates deeply into the resultant foam where such a high internal temperature is reached, so that it usually takes place that the resultant open cell rigid polyurethane foam readily carbonizes. Thus, the resultant foam readily scorches.